An observation campaign called TANUKI2011 which is simultaneous observations of the Middle and Upper atmosphere (MU) radar and radiosonde was implemented on September 2011 at Shigaraki MU observatory in Japan. In this campaign, MU radar observed convection below midlevel cloud base where the atmosphere was convectively unstable and the vertical shear was weak. In order to investigate the origin of the convection we performed high resolution three dimensional numerical simulations initialized with thermodynamic parameters derived from the radiosonde data. Simulations showed that convection rolls appeared below cloud base where the convective instability was enhanced by sublimation of snow. The Rayleigh number exceeded the critical value before the convection peaked and the wave fronts of the roll convection were parallel to the vertical shear direction. We compared the results of simulation with observations and found that parameters of the convection (such as vertical wind speed, turbulent kinetic energy, layer depth, period) calculated by the simulation agreed well with MU radar observations. The good agreement and the results of simulations suggest that the convection must be Rayleigh-Bénard convection which is generated by sublimation cooling of snow below cloud base.